A drill bit is a cutting tool for drilling steel products and the like. FIG. 1 shows an exemplary structure of a twist drill bit. The twist drill bit is formed by a cutting portion 1 for performing the drilling, and a shank 2 which is not concerned with the cutting but mainly adapted to discharge chips and is mounted in a chuck of a cutting machine such as a drilling machine.
In general, materials for drills are made of high-speed steel and cemented carbide. The high-speed steel, which is excellent in toughness but inferior in wear resistance, is not suitable for high-speed cutting. On the other hand, cemented carbide, which is excellent in wear resistance and tool accuracy characteristics but brittle, may cause breakage when the same is used in a machine tool having low rigidity, for example.
In order to improve such materials, there has been proposed a structure attained by coating a cutting portion of high-speed steel with hard TiN and a structure attained by brazing a cutting portion of cemented carbide.
In recent years, there have been proposals of a structure attained by brazing different cemented carbide materials (P30 and D30) for improving the wear resistance and the toughness as disclosed in Japanese Utility Model Laying-Open No. 58-143115 (1983), or metallurgically integrating/connecting such different materials as disclosed in Japanese Utility Model Publication No. 62-46489 (1987), a drill bit of a double structure whose central and outer peripheral portions are made of different cemented carbide materials in consideration of the different characteristics required for such portions as disclosed in Japanese Patent Laying-Open No. 62-218010 (1987), or a method of attaining such a double structure by injection molding as disclosed in Japanese Patent Laying-Open No. 63-38501 or 38502 (1988). There has also been proposed a structure attained by preparing a material for a drill bit of a cermet Ti-base cemented carbide in order to improve the adhesion resistance, as disclosed in Japanese Patent Laying-Open No. 62-292307 (1987).
On the other hand, diamond, which has a number of excellent characteristics such as an extremely high hardness, a chemical stability, and a high thermal conductivity, has been widely applied as a hard material itself, or for coating a hard material with diamond or with carbon having a diamond-structure. Examples of conventional diamond tools are cutting tools such as a throwaway tip, a drill bit, a microdrill bit and an end milling cutter for working various light metals and alloys thereof or plastic materials, and various wear resistant tools such as a bonding tool employed for packaging a chip-type component.
Several methods are known for preparing or forming artificial diamond which is applied to manufacturing of such tools. These methods involve .alpha. wave plasma CVD, RF-plasma CVD, EA-CVD, magnetic field .alpha. wave plasma CVD, RF heat plasma CVD, DC plasma CVD, DC plasma jet CVD, filament heat CVD, and a combustion method. These methods can be used for the formation of diamond coating layers from vapor phases. These are prevailing methods of preparing diamond-coated hard materials.
A hard base material useful to be diamond-coated to provide a high adhesion strength between the hard base material and the diamond coating is selected to have a thermal expansion coefficient identical to the thermal expansion coefficient of diamond. Japanese Patent Laying-Open No. 61-291493 (1986) proposes using as a hard base material a sintered body which is mainly composed of Si.sub.3 N.sub.4 or SiC.
In general, an Al-Si alloy or a printed circuit board has been perforated by a WC-based cemented carbide drill bit, or a drill bit of surface-coated WC-based cemented carbide which is prepared by coating the surface of such a WC-based cemented carbide drill bit with a single ply or with multi-plies hard coating layer or layers of a carbide, a nitride and/or a carbo-nitride of Ti, Zr or the like in a thickness of 0.2 to 20 .mu.m by chemical vapor deposition such as ordinary CVD or physical vapor deposition such as ion plating or sputtering. In particular, a printed circuit board has been perforated by a so-called microdrill, which is formed of a material similar to the above but having a relatively small diameter.
In recent years, however, a labor reduction and a speed increase have been required for perforation of an Al-Si alloy and a printed circuit board, under strict drilling conditions. Under such severe conditions, the useful life of the aforementioned WC-based cemented carbide drill bit has ended in an extremely short time when the same is used for perforating an Al-Si alloy or a printed circuit board, due to significant wear. Also in the drill bit made of surface-coated WC-based cemented carbide, it is impossible to attain a desired cutting performance since its hard coating layer may be significantly worn in a short time to cause separation of the coating or chipping.
Although an attempt has been made to coat the surface of a WC-based cemented carbide drill bit with a diamond coating layer by a well-known low-pressure vapor phase synthesizing method, such a technique has not yet been put into practice for the following reasons:
(1) Graphite having a low wear resistance is apt to precipitate on the surface of Co contained in the cemented carbide. If the Co content is reduced in order to prevent this, the toughness of the cemented carbide is lowered to easily cause chipping during cutting.
(2) Since linear expansion coefficients of diamond and the cemented carbide are extremely different from each other, the adhesion strength of the diamond coating layer is reduced due to residual stress, such that the coating layer itself is separated from the base material when its thickness exceeds 20 .mu.m. Such separation is also easily caused during cutting even if the coating layer has a small thickness.
On the other hand, material requirements for printed circuit boards have been so varied that there has been developed an extremely hard material, which is mainly composed of ceramic resin etc., having a Vickers hardness exceeding 100.
To this end, the aforementioned microdrill bit of WC-based cemented carbide (Vickers hardness: about 1500) or surface-coated WC-based cemented carbide (Vickers hardness: about 2000 to 2500) is so insufficient in hardness that it is impossible to attain a high wear resistance. Further, since such a microdrill bit has an elongated configuration with a small diameter of not more than 5 mm, or not more than 3 mm in general, chipping may be caused in a short time or the hard coating layer may be separated or significantly worn if the drill bit is used under severe operating conditions. In addition, the material, such as ceramic resin, for the printed circuit board is easily deposited onto the tip of a cutting portion of the drill bit. Thus, the conventional microdrill bit cannot satisfy necessary cutting performance requirements.
In the case of a drill bit having a cutting portion and a shank which are integrated with or inseparably connected with/fixed to each other, the following problems are encountered:
A cutting portion and a shank of a drill bit are used under different load conditions. Therefore, different characteristics are required for the respective parts of such a drill bit. For example, wear resistance and adhesion resistance are required for a tip of the cutting portion, while toughness for maintaining strength of the tool is required for the shank. Further, the tip of the cutting portion must have different characteristics in its central portion and in its outer peripheral portions thereof, since these portions are driven at different cutting speeds.
When a drill bit whose cutting portion is coated in order to satisfy such complicated requirements for the characteristics, is resharpened for general use, the coating layer is inevitably separated at least from a front flank side, and hence most of the coating effect is lost.
On the other hand, a drill bit which is formed by brazing cemented carbide to its cutting portion, cannot be used for deep hole drilling of a hard to cut material. If the shank is made of steel, there is a significant difference between the thermal expansion coefficients of the steel and the cemented carbide forming the cutting portion, to easily cause splitting or cracking during the brazing.
In recent years, a cemented carbide material for the shank of a drill bit has been brought into a coarse grain or high binder phase state, in order to improve the toughness of the shank. In that case, however, the strength of the material is reduced or the distortion of an elastic limit is reduced, and hence the shank is undesirably broken during perforating, due to vibration of a workpiece, due to an unstable rotation of a cutting machine, or the like.
On the other hand, in a drill bit of diamond-coated cemented carbide, the coating is separated in an initial operating stage due to an insufficient adhesion strength between the diamond coating layer and the cemented carbide. Hence, it is impossible to improve the wear resistance in the manner described.
When a twist drill bit having a base material of a silicon nitride based ceramic sintered body, is coated with diamond, the possibility of separation of the diamond coating layer is greatly reduced. However, such a twist drill bit tends to break under severe cutting conditions, due to an insufficient strength of the sintered body. This drawback also applies to a diamondcoated drill bit made of a base material of ceramic such as alumina or SiC.
While a drill bit formed by inseparably and integrally connecting the aforementioned cutting portion and shank, can be continuously used by resharpening the cutting portion after every prescribed operating time, the frequency of such resharpening is restricted, and the cost is increased due to the time needed for resharpening. Further, the sharpness and the tool life are varied depending on the conditions of the resharpening operation. In addition, it is necessary to successively correctly comprehend the length of the drill bit in response to a numerical control and the automation of a cutting machine to which the drill bit is applied. Thus, the length of the drill bit must be measured by a complicated operation every time the same is resharpened.